Highway Fuel Consumption

1. Definition and Functional Positioning

Highway fuel consumption refers to the fuel consumption measured when a vehicle cruises steadily at relatively high speeds on clear, open roads such as highways or expressways, expressed in litres per 100 kilometres (L/100km). Its primary purpose is to reflect a vehicle's fuel efficiency under sustained high-speed driving conditions. Compared to urban and combined fuel consumption figures, highway fuel consumption is typically the lowest, serving as an essential benchmark for consumers assessing long-distance travel costs.

2. Test Standards and Driving Cycle Characteristics

Highway fuel consumption figures are determined by the specific high-speed driving phases prescribed across different test cycles.

3. Relationship with Urban and Combined Fuel Consumption

Highway fuel consumption is generally significantly lower than urban and combined figures. This is attributable to:

• High engine operating efficiency: During highway cruising, the engine operates within its optimal efficiency band for extended periods (typically 2,000–3,000 rpm), delivering higher thermal efficiency.

• Aerodynamic drag as the primary resistance: Energy expended to overcome acceleration and rolling resistance is reduced, kinetic energy fluctuations are minimal, and overall energy conversion efficiency remains high.

• Zero idling losses: Driving remains continuous, eliminating the fuel wastage associated with stationary idling.

Typical Comparison Example: Under identical test standards, a vehicle rated at 7.4 L/100km on the WLTC combined cycle might return 9.0 L/100km in urban driving, yet drop to 6.0–6.5 L/100km on the highway. In real-world driving at cruising speeds of 110–120 km/h, actual fuel economy typically aligns closely with official extra-urban or highway figures.

4. Key Factors Influencing Highway Fuel Consumption

• Vehicle speed: Once vehicle speed exceeds 80 km/h, aerodynamic drag climbs exponentially; at 120 km/h, drag is more than double that at 80 km/h, causing fuel consumption to rise sharply.

• Aerodynamic design: A lower coefficient of drag (Cd value) reduces the power required to overcome wind resistance at higher speeds. Sedans generally outperform SUVs in this aspect, while design elements like flush door handles and active grille shutters further reduce drag.

• Transmission gear ratio calibration: Vehicles geared with a taller top gear ratio maintain lower engine revs when cruising at 120 km/h, resulting in superior fuel economy.

• Vehicle weight and tyre rolling resistance: Higher vehicle kerb weight and elevated tyre rolling resistance coefficients will inevitably push highway fuel consumption upward.

5. Development Trends

• Impact of test standard transitions on highway consumption figures: With WLTC comprehensively replacing NEDC, test durations and speed profiles for highway testing have changed noticeably. The WLTC cycle peaks at 131 km/h with a substantially longer high-speed duration, leading to officially rated highway fuel consumption figures that are generally higher than NEDC ratings, yet far more reflective of real-world long-distance highway driving.

• Aerodynamic optimisation: EV range sensitivity in the electrification era has compelled carmakers to refine body aerodynamics, and the widespread adoption of ultra-low Cd designs continues to reduce high-speed energy consumption.

• Highway fuel economy optimisation in Hybrid and PHEV models: Hybrid powertrains utilise electric motor assist to keep the internal combustion engine operating in its sweet spot during highway cruising, cutting fuel consumption further. On expressways, due to the electric motor's high-efficiency characteristics and aerodynamic properties, pure electric models still retain a substantial energy efficiency advantage over conventional ICE vehicles.

• ADAS and predictive cruise control: Smart energy management systems integrating navigation map data and sensor fusion can anticipate gradients and upcoming traffic flow changes, proactively modulating power delivery to optimise high-speed cruising energy efficiency.